This invention relates to a fibre optic cable, to a method of sensing spatial variations and/or temperature variations in a structure using a fibre optic cable, and to a device for sensing spatial variations and/or temperature variations in a structure, comprising a fibre optic cable.
The use of fibre Bragg gratings in sensors is well known. U.S. Pat. No. 4,761,073 (incorporated herein by reference) describes a spatially resolving fibre optic cable strain gauge which utilises fibre Bragg gratings, and it is also known to use similar fibre Bragg grating sensors to monitor variations in temperature. Variations are able to be sensed as the period and the effective refractive index of a fibre Bragg grating are altered as a result of a change in strain or a change in the ambient temperature in the locality of the fibre Bragg grating. However, fibre Bragg grating sensors forming the prior art suffer from a lack of spatial accuracy, and as a result the ability to precisely locate the source of strain and/or heat is compromised.
Fibre Bragg gratings are formed in photosensitive fibre optic cables by creating a periodic variation in the refractive index of the core of the fibre optic cable, which acts as a grating to reflect incident light. The wavelength of this reflected light, known as the Bragg wavelength λB, is dependent on the grating period and the effective refractive index of the fibre Bragg grating, according to the equationλB=2neffΛ,where neff is the effective refractive index of the fibre optic cable and Λ is the period of the fibre Bragg grating.
This Bragg wavelength is known to be affected as a consequence of localised spatial and/or temperature variations in the vicinity of the fibre Bragg grating. Fibre Bragg gratings have predictable, well defined responses to spatial and/or temperature variations. These responses are known to be approximately linear at and above room temperature, and fibre Bragg grating sensors operate by measuring the wavelength shift of the Bragg wavelength in response to the aforementioned spatial and/or temperature variations.
The Bragg wavelength shift ΔλB in response to spatial and/or temperature variations in the fibre optic cable in the vicinity of the fibre Bragg grating is brought about by a change in the grating period, and is found by differentiating the above equation to account for changes in the length of the fibre optic cable and/or the temperature in the vicinity of the fibre Bragg grating. This results in
      Δ    ⁢                  ⁢          λ      B        =            2      ⁢              (                              Λ            ⁢                                          ∂                                  n                  eff                                                            ∂                l                                              +                                    n              eff                        ⁢                                          ∂                Λ                                            ∂                l                                                    )              +          2      ⁢                        (                                    Λ              ⁢                                                ∂                                      n                    eff                                                                    ∂                  T                                                      +                                          n                eff                            ⁢                                                ∂                  Λ                                                  ∂                  T                                                              )                .            
There are currently two approaches to making sensors using fibre Bragg grating technology. The simplest approach is to write a plurality of identical fibre Bragg gratings (i.e. gratings with the same grating pitch) along the length of a fibre optic cable. Incident light is reflected by each of these fibre Bragg gratings, the Bragg wavelength of each reflection being the same due to the equality of the fibre Bragg gratings. Spatial and/or temperature variations in the locality of the fibre optic cable would result in a shift of the Bragg wavelength of the light reflected by one or more affected fibre Bragg gratings. This change can be sensed, and hence it is possible to detect that a spatial and/or temperature variation has occurred.
The approximate position of the spatial and/or temperature variations can be obtained by optical time domain reflectometry. In this technique, a very short pulse of light (of the order of 1 ns) is injected into the fibre optic cable and the reflected spectrum measured as a function of time after the pulse injection. This approach is limited in that, although spatial and/or temperature variations in the locality of the fibre optic cable are detectable, there is no provision for the precise location of such variations, given that the resolution of the position information is practically only of the order of a few meters, meaning that only fibre Bragg gratings that are several meters apart can be distinguished.
An alternative approach is to write a plurality of fibre Bragg gratings along the length of a fibre optic cable, each fibre Bragg grating having a unique grating pitch. Incident light is reflected by each of these fibre Bragg gratings, the Bragg wavelength of each reflection in this case being characteristic of a particular fibre Bragg grating. Spatial and/or temperature variations occurring in a particular vicinity will affect one or more of the fibre Bragg gratings in that vicinity, leading to a shift of the Bragg wavelength of the light reflected by the affected fibre Bragg grating or gratings. Analysis of the reflected light patterns therefore enables detection of the occurrence of a spatial and/or a temperature variation. The location of the variation along the fibre optic cable is also detectable by determining from which particular fibre Bragg grating or fibre Bragg gratings the shifted Bragg wavelength emanates. However, the Bragg wavelengths of the fibre Bragg gratings must in this case be sufficiently far apart so that the shift in the Bragg wavelength over the operating range of any grating does not overlap the different Bragg wavelength associated with another fibre Bragg grating. This provides a major restriction on the number of fibre Bragg gratings that can be interrogated with one sensor, introducing a degree of insensitivity to the system as the total number of Bragg wavelengths able to be sensed is severely limited to around 10 with the broadband light sources and spectrometers currently available. This means that a sensor with a fibre optic cable 10 m long will only have one fibre Bragg grating per meter. This creates large gaps where there is no sensitivity as local heating between two fibre Bragg gratings would not be detected. This is unsuitable for applications where a localised “hot-spot”, only a few centimeters long, must be able to be detected anywhere along a sensor which may be many meters long.
The insensitivity described above is caused by a practical limit on the length a single fibre Bragg grating can have. Also, larger fibre Bragg gratings typically have a lower reflectivity per unit length, meaning that the reflection returned is an average of the total length of the fibre Bragg grating. In this case, the reflection from a localised “hot-spot” on the fibre Bragg grating would be relatively weak and difficult to detect.
The present invention offers an improvement to the approaches described above.